Such an engine valve operating system is already known from Japanese Patent Application Laid-open No. 2004-036560. In this valve operating system, a rocker arm for driving the valve is pivotally supported on an engine body through two links, so that only the lift amount is changed continuously without changing the opening angle of the valve by driving one of the links by a valve-operating cam to swing the rocker arm and by moving the position of a fulcrum of the one link on the side of the engine body.
In the engine valve operating system, a predetermined tappet clearance is provided between an adjusting bolt mounted in the rocker arm and a stem end of the valve. For this reason, if the rocker arm is started to be operated to open the valve, the valve is not lifted immediately, and is started to be lifted after the rocker arm is raced in an amount corresponding to the tappet clearance to bring the adjusting bolt into abutment against the stem end.
FIG. 10 shows valve lift curves for the conventionally known valve operating system. In this valve operating system, the valve lift can be changed, but the following problem is encountered: The tappet clearance is changed neither at a high valve lift nor at a low valve lift. For this reason, the ratio of the tappet clearance to the valve lift amount in a low valve lift state is too large, and the slight dispersion of the tappet clearance largely influences an amount of air drawn and thus, lots of labor and time are required for the fine adjustment of the tappet clearance. Especially, in such an engine in which the valve lift can be changed, the valve is brought into a state of a low lift equal to or smaller than 1.5 mm during the majority of an operating time and hence, the influence of the dispersion of the tappet clearance is not negligible.
In the above-described system, a speed of seating of the valve (see an inclination L of the valve lift curve) at the low valve lift is large, as compared with a speed of seating of the valve (see an inclination H of the valve lift curve) at the high valve lift, as is apparent from a graph in FIG. 10. Therefore, there is a possibility that the seating noise of the valve might be increased, or adversely influences the durability of a valve-operating mechanism, especially at the low valve lift. In order to avoid this, if the profile of a cam is changed to provide a reduction in seating speed of the valve at the low valve lift, then the opening and closing speed of the valve at the high valve lift is reduced, resulting in a problem that the engine cannot exhibit a desired power output.